It is a common practice to acidize subterranean formations in order to increase the permeability thereof. For example, in the petroleum industry, it is conventional to inject an acidizing fluid into a well in order to increase the permeability of a surrounding hydrocarbon-bearing formation and thus facilitate the flow of hydrocarbonaceous fluids into the well from the formation or the injection of fluids such as gas or water, from the well into the formation. Such acidizing techniques may be carried out as "matrix acidizing" procedures or as "acid-fracturing" procedures.
In acid fracturing the acidizing fluid is disposed within the well opposite the formation to be fractured. Thereafter, sufficient pressure is applied to the acidizing fluid to cause the formation to break down with the resultant production of one or more fractures therein. An increase in permeability thus is effected by the fractures formed as well as by the chemical reaction of the acid within the formation.
In matrix acidizing, the acidizing fluid is passed into the formation from the well at a pressure below the breakdown pressure of the formation. In this case, increase in permeability is effected primarily by the chemical reaction of the acid within the formation with little or no permeability increase being due to mechanical disruptions within the formation as in fracturing.
In yet another technique involving acidizing, the formation is fractured. Thereafter, an acidizing fluid is injected into the formation at fracturing pressures to extend the created fracture. The acid functions to dissolve formation materials forming the walls of the fracture, thus increasing the width and permeability thereof.
In most cases, acidizing procedures are carried out in calcareous formations such as dolomites, limestones, dolomitic sandstones, etc. One difficulty encountered in the acidizing of such a formation is presented by the rapid reaction rate of the acidizing fluid with those portions of the formation with which it first comes into contact. This is particularly serious in fracture acidizing where pressures, high formation temperatures, and high acid solubility greatly limit the amount of formation that can be contacted by unreacted ("live") acid before it spends on the formation rock. As the acidizing fluid is forced from the well into the propagating fracture, the acid reacts rapidly with the calcareous material immediately adjacent to the fracture. Thus, the acid becomes spent before it penetrates into the formation a significant distance from the fracture. For example, in fracture acidizing of a limestone formation, it is common to by-pass vugs ("high pores") as the fracture often propagates too fast to interconnect with the vugs. Therefore, the porosity of the vuggy formation is not sufficiently increased because many of the vugs are not interconnected. As a result, hydrocarbonaceous fluids contained in the vugs are not removed and the formation or reservoir is not sufficiently drained. This, of course, severely limits the increase in productivity or injectivity of the well.
In order to increase the penetration depth, it has heretofore been proposed to add a reaction inhibitor to the acidizing fluid. For example, in U.S. Pat. No. 3,233,672 issued to N. F. Carpenter, there is disclosed an acidizing process in which an inhibitor, such as alkyl-substituted carboximides and alkyl-substituted sulfoxides, is added to the acidizing solution. Another technique for increasing the penetration depth of an acidizing solution is that disclosed by U.S. Pat. No. 3,076,762 issued to W. R. Dill, wherein solid, liquid, or gaseous carbon dioxide is introduced into the formation in conjunction with the acidizing solution. The carbon dioxide acts as a coolant, thus retarding the reaction rate of the acid with the formation carbonates. Also, the carbon dioxide is said to become solubilized in the acidizing solution, thus resulting in the production of carbonic acid which changes the equilibrium point of the acid-carbonate reaction to accomplish a retarding effect.
An additional procedure disclosed in U.S. Pat. No. 2,850,098 issued to Moll et al. involves the removal of contaminants from a water well and the adjacent formation through the injection of gaseous hydrogen chloride. Still another technique for acidizing a calcareous formation is disclosed in U.S. Pat. No. 3,354,957 issued to Every et al. In this process liquid anhydrous hydrogen chloride is forced from a well into the adjacent formations. The liquid hydrogen chloride vaporizes within the formation and the resulting gas dissolves in the formation to form hydrochloric acid which then attacks the formation.
From these teachings it is apparent that there are numerous limitations to present methods of fracture acidizing and diverting techniques. One major limitation is that the acid reaction begins in a propagating fracture with no substantial resistance. Due to the speed of the propagating fracture, some areas of the formation near the fracture receive no acid to increase the porosity of the formation.
Therefore, what is needed is a method to improve fracture acidizing by increasing fracture communication with large hydrocarbonaceous fluid containing pores so as to increase the porosity of a formation thereby substantially increasing the drainage of hydrocarbonaceous fluids from the formation.